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After depleting the hydrogen in its core, an intermediate-mass star contracts and heats up until it sets off the fusion of helium into heavier elements. Helium fusion provides a second stable phase, covering the last 10 percent of the star’s life. The increase in energy production puffs out the star’s atmosphere, resulting in a highly luminous red giant star. When the Sun reaches this stage, it will engulf the orbit of Venus and evaporate Earth’s oceans.